Micro-quakes, big questions
“A magnitude-1.6 earthquake is exceedingly small and we wouldn’t have detected it without the densification of our sensor network in Northland that occurred in 2022-23. Areas of Northland now have network density comparable to the East Coast (Hikurangi Subduction Zone) and Southern Alps (Alpine Fault) plate boundary regions, where recorded earthquake counts are in the 1000s per year.”
Since this network rolled out, six earthquakes in Northland, or about two a year, have been detected.
“And that’s not for a lack of trying,” Taylor said, adding that the quake gives some new insight into the dormant faults in Northland.
“The earthquake tells us that just because you’re far away from the plate boundary doesn’t mean there isn’t stress accumulating in the crust.”
Taylor said Earth Sciences New Zealand doesn’t categorise any faults in Northland as active, but there are plenty of faults in Northland from when it was active tectonically some 10 million years ago.
“Faults developed then to accommodate compression, like what is happening on the East Coast today. This earthquake may have happened on one of the old faults or just somewhere in the crust.”
Despite the quake this week, Northland’s earthquake hazard hasn’t changed significantly.
“We expect earthquakes everywhere and this just reminds us that they can happen in Northland, as they did in 1963-1964 around Mangōnui and Peria west of Whangaroa, when a period of seismicity produced a magnitude-4.8 and many smaller earthquakes. Earthquakes have also occurred in the region sporadically since then, which is what we expect for a stable region like Northland.”
Looking forward, he said the trend of sporadic, small earthquakes is likely to continue as a result of the many old faults in Northland.
“More earthquakes will happen in Northland, but I expect the trend of the last ~80 years will continue for a few million more at least. Unfortunately, that doesn’t mean a larger earthquake than what we have seen isn’t possible.”
“There are many old faults in Northland and, as the recent earthquake has shown, there is still enough stress building in the crust to break it. The good news is it could be a long time before such an earthquake happens, if it ever does.”
Why Northland is different
Northland’s distance from the plate boundary is what sets it apart from other parts of the country that are more seismically active.
“Northland is sort of ‘behind’ all the action tectonically, especially in the Far North, because the compression and extension from plate tectonics is being accommodated elsewhere,” Taylor said.
He said people in the region should be more aware about earthquake risk, despite the region’s low activity.
“It’s something everyone should be aware of and know what to do in an earthquake and what they might need to do in the time afterwards.”
Drop, cover and hold during a large earthquake, Taylor advised.
If it is long or strong, get gone, the official advice suggests.
If you are near the coast or a lake and feel a strong earthquake that makes it hard to stand up or a weak rolling earthquake that lasts a minute or more, move immediately to the nearest high ground or as far inland as you can, out of tsunami evacuation zones.
